Navigational lighting systems in many locations are not easily accessible, must operate with high reliability in order to provide safety, and be autonomously powered. The electrical power, which is generally batteries, and the lamps are conserved by operating only at night, and are turned off in the daytime by light sensors.
In order to avoid possible ambiguity to mariners, it is desirable to have all lanterns marking a channel, hazard, or structure to light or flash at the same time at dusk, and to extinguish simultaneously at dawn. This is difficult to accomplish by independent light sensing devices connected to each of the lanterns because (1) available light sensors are difficult to calibrate, compensate, and do not track each other well, (2) in extreme northern and southern latitudes, the transition from night to day may be quite slow, thereby causing lanterns only slightly out of calibration to light and extinguish far apart from each other in time, and (3) shading of the light sensor by other structures causes orientation sensitivity and sensitivity to the sun angle.
In U.S. Pat. No. 3,781,853, a navigational light system is disclosed in which all of the lanterns flash in unison and are actuated by a synchronizing signal initiated by the first lantern to be actuated. That system synchronized all of the navigational lights and insured that in the event one of the lanterns failed, the remainder of the lanterns would operate.
However, the present systems utilize a single light measuring threshold point at the output of the light sensors to define the boundary between day and night. If the output of the sensor is above the threshold (day condition), flashing of the lantern is inhibited, and if below the threshold (night conditions), flashing is enabled. All of the lanterns send synchronizing pulses at the beginning of each flash code sequence to all of the lanterns to actuate all of the lanterns that are not inhibited.
The present invention is directed to an improved method and apparatus for synchronizing a plurality of navigational lanterns by compensating for the fact that the independent light sensors connected to each of the lanterns respond differently to the light levels as well as allow the system to operate even in the event of a light level sensor failure.